This invention relates to a spiral antenna with constant beamwidth over an extended frequency range and in particular to a center-fed two-arm spiral antenna with its arms incorporating a design of semi-lumped element to form low pass filters which attenuate undesired excess currents.
Spiral antennas have been in use extensively as electromagnetic radiator and various geometries have been investigated, including square, hyperbolic and elliptic. Archimedian and equiangular (or logarithmic) spiral antennas, however, seem to be the most commonly used; the equiangular spiral is used in both planar and conical configurations, while the Archimedean is primarily used in the planar configuration.
Multi-arm spiral antennas have also been investigated for generating sum and difference patterns by choosing appropriate phase relationships among the arms. Even though the spiral is inherently circularly polarized in one sense only, such as the right-hand circular or the left-hand circular depending on whether the spiral antenna arms are wound in clockwise or counter-clockwise direction, experiments have been conducted to achieve both senses of circular polarization from a single spiral. A multi-arm spiral antenna for operation with dual senses of circular polarization was disclosed in Ingerson, U.S. Pat. No. 3,681,772 with arms having log-periodically scaled width variations to produce local reflection regions. A method of using choke elements that resonate at predetermined frequencies to eliminate or minimize undesired radiation was disclosed in Lamberty et al., U.S. Pat. No. 4,243,993. Performance of center-fed two-arm spirals, however, has remained unchanged over a long period of time. The two-arm spiral antennas are used individually, in monopulse direction finding system made of several spirals, or as array elements. In general, the individual spiral antenna pattern is of primary concern in each of these cases.
The spiral antenna is a very broadband device, but it is not truly frequency-independent because the radiation pattern varies with frequency when the antenna is designed to cover a multi-octave frequency bandwidth. This variation is attributable to the conclusion by the band radiation theory that current flowing outward from the center of the spiral is radiated when the current reaches a band or a ring having circumference equivalent to one wavelength. The unradiated current continues to flow outward until another ring of proper diameter is reached, at which point radiation takes place again. This secondary radiation interferes with the primary radiation, thus causing variations in the pattern beamwidths.
It is therefore an object of this invention to provide a broadband two-arm spiral antenna with constant beamwidth over unlimited bandwidths.
It is another object of this invention to provide a two-arm center-fed Archimedean spiral antenna which eliminates undesired radiation characteristics such as secondary radiation.
It is a further object of this invention to provide a two-arm Archimedean spiral antenna with its arms forming low pass filters which attenuate the excess currents that flow beyond the primary radiation ring of the band radiation theory.
These and other objects are achieved in accordance with this invention by a center-fed two-arm Archimedean spiral antenna with each arm designed with abrupt width variations such that normal-width sections and reduced width sections occur alternately along its length. The lengths of these sections are determined by a known method of designing Tchebyscheff serial-L shunt-C, ladder low pass filters using semi-lumped elements.